1. Field of the Invention
The present invention relates to a measurement instrument in the field of medical electronics. More particularly to a plethysmograph for determining and registering variations in the size of an organ or limb and in the amount of blood present or passing through it.
2. Description of the Prior Art
The use of plethysmograph instruments in the medical field has been known for a considerable period of time. Generally, light rays are transmitted into a finger or earlobe of the human body. An output signal is derived from either the light rays transmitted through or reflected off the body. The resulting plethysmogram is a plot showing a train of waves representative of the amount of blood present in that region of the human body.
Reference is made to the article, "Photoelectric Determination of Arterial Oxygen Saturation in Man", by Wood et al in the Journal of Laboratory and Clinical Medicine, Vol. 34, 1949. In that article, an oximeter is described wherein a light source generates light in the infrared region and in the red region. Light wave signals that are transmitted through the pinna of the human ear are photoelectrically converted into a first and second output signal to determine the oxygen saturation in the arterial blood. The Herczfeld et al U.S. Pat. No. 3,704,706 discloses an apparatus for the detection of pulse repetition rate and oxygenation of blood flow by the direction of light through a patient's finger. Additional examples of prior art can be found in the Liston U.S. Pat. No. 2,640,389 and the Polanyi et al U.S. Pat. No. 3,628,525.
Common problems have existed in the subjective calibration of the instruments to a particular patient and the sensitivity of the instruments to various conditions of the skin. One of the more important regions of the human body to monitor the normality of blood flow relates to the brain, since serious damage is capable of happening in a relative short period of time. Thus, the prior art is still seeking improvements in plethysmograph devices.